Review
Memory formation, consolidation and transformation

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Abstract

Memory formation is a highly dynamic process. In this review we discuss traditional views of memory and offer some ideas about the nature of memory formation and transformation. We argue that memory traces are transformed over time in a number of ways, but that understanding these transformations requires careful analysis of the various representations and linkages that result from an experience. These transformations can involve: (1) the selective strengthening of only some, but not all, traces as a function of synaptic rescaling, or some other process that can result in selective survival of some traces; (2) the integration (or assimilation) of new information into existing knowledge stores; (3) the establishment of new linkages within existing knowledge stores; and (4) the up-dating of an existing episodic memory. We relate these ideas to our own work on reconsolidation to provide some grounding to our speculations that we hope will spark some new thinking in an area that is in need of transformation.

Highlights

► Understanding memory formation requires analysis of the various representations that result from experience. ► Memory is not just formed, but also reformed, or transformed, after initial encoding. ► Transformation includes changes in both the strength (or weakness) or memory traces, and the updating of existing traces through the integration of new information. ►Memories are inherently dynamic, changing both with the passage of time, and as a function of new experiences.

Section snippets

The nature of memory formation

The textbook view of memory formation goes something like that shown in Fig. 1 below.

That is, we experience an event, some aspects of this event get encoded, this encoding initiates a series of processes, typically labeled “consolidation”, that take time, leading to a permanent memory trace. Memory consolidation plays a key role in this sequence, as it determines both what will be preserved after initial encoding, and how long this process takes. Once consolidation is over, by definition, the

Problems with the textbook view

The problem with this view of memory formation is that it appears to be wrong. As Fig. 2 shows, when an apparently fixed memory is reactivated, some process is initiated that renders what initially seemed fixed once again labile (Nader et al., 2000; see also Lewis et al., 1968 and Misanin et al., 1968). A reactivated “consolidated” memory, that has become labile, is open to change in ways that a non-reactivated “consolidated” memory is not. This change can take any of several forms, including

What next

Our view is that the facts alluded to above represent not just a challenge to the classic story concerning memory formation. They also create an opportunity to re-evaluate some basic assumptions about memory. The remainder of this paper is devoted to doing exactly that, in a spirit of what we hope is productive speculation. We consider the following questions:

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    If memory consolidation is not about permanently fixing a memory trace, what is it about?

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    What forms of memory transformation are

What is memory consolidation all about?

In contrast to the standard textbook story, we assume that consolidation is not only, or even primarily, about permanently fixing an isolated memory trace. Taking into account the points raised above about the nature of memory, we see memory consolidation as accomplishing a number of different things, depending on what aspect of memory one is looking at.

Consider what happens when an organism experiences a novel episode, which we can view as consisting of a set of entities (individuals, objects)

What forms of memory transformation are possible? How are they instantiated in different brain systems? And what role does sleep play in the transformation process?

Following on from the analysis above, what can we say about the kinds of transformations that occur during memory consolidation and reconsolidation?

It would be instructive to work through the various stages of memory transformation in terms of an actual example for which some empirical data exist. Fig. 4 portrays a paradigm we have recently been using to study episodic memory updating in humans. In brief, subjects learn a list of twenty common everyday objects, then two days later come back and

Acknowledgement

We gratefully acknowledge support from the National Science Foundation (BCS-0743988) for the research described herein.

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